This invention relates to apparatus and methods for measuring, monitoring and/or controlling the characteristics of a medium which may be contacted with a probe. The probe is designed for use with conventional frequency and time domain impedance analyzers.
Interest in the use of electrical measurements to characterize materials has existed for over fifty years. Many excellent books reviewing this literature have been written, i.e. Hedvig, P., Dielectric Spectroscopy of Polymers, John Wiley, New York, 1977; Hill, N., Vaughan, W., Price, A., and Davies, M., Dielectric Properties and Molecular Behavior, Van Nostrand Reinhold, New York, 1969; McCrum, N., Read, B., and Williams, G., Anelastic and Dielectric Effects in Polymeric Solids, John Wiley, London, 1967; North, A., J. of Polymer Science Symposium 50. 345-358, 1978; Wada, Y., Dielectric and Related Molecular Processes 3, 143-175, 1977; Karasz, F., Dielectric Properties of Polymers, Plenum Press, New York, 1972; Read, B. E. and Dean, G. D., The Determination of Dynamic Properties of Polymers and Composites, John Wiley, New York, 1978, and May, C. A., Chemorheology of Thermosetting Polymers, American Chemical Society Symposium Ser., 227, 1983. The extensive use of electrical measurements, primarily utilizing dielectric techniques, to characterize materials has led to a current interest in using electrical measurements to monitor cure processes in resins. The reason for this interest is that there are very few other techniques for convenient and continuous monitoring of the cure process through a wide range of resin viscosity, i.e. less than 10.sup.2 to more than 10.sup.9 poise.
Until the last several years, research on the use of electrical measurements has often fallen short of expectations. While reasons for this are in most cases complex, usually they were related to inadequate instrumentation, i.e. the use of instrumentation such as sensors not designed specifically for the purpose, failure to make measurements over a wide range of frequencies, not understanding the molecular basis for the signal in terms of dipolar and ionic phenomena, and lack of an integrated approach to interpret the electrical measurements in a manner correlatable to other chemical characterization measurements.
The effective and successful use of dynamic electrical measurements for cure cycle monitoring requires an extensive effort involving basic research on the chemistry of each resin or other medium using a variety of characterization techniques, which are then correlated with dynamic electrical measurements made over a wide range of frequencies. Only with this background information is it possible for electrical measurements to be most effectively used to monitor the cure process or other reaction and to provide the input for a complete mathematical processing model for quality assurance cure cycle monitoring as well as closed loop "smart" cure cycle control.
As indicated above, convenient and accurate measurements of electrical characteristics of chemicals undergoing chemical reactions have not been easily made in the past or utilized conventionally to monitor and/or correlate the chemical and physical characteristics of the reaction products with them. I have observed particularly that the complex permittivity of a polymerizing resin has a particular relationship to the progress of the polymerization reaction, but until now the art has not developed the apparatus and techniques necessary to exploit that relationship over the wide range of frequencies and the wide variation in the magnitude of the complex permittivity required for a useful correlation.
For example, a description of a rather elaborate system for controlling polymerization process variables is given in U.S. Pat. No. 4,448,943, but it utilizes a "slit die into which is incorporated a parallel plate capacitance cell" for the measuring device. While the parallel plate capacitance cell will provide a gross measurement of capacitance, it has certain disadvantages, compared to my planar interdigitated device including an inability to maintain a controlled spatial relationship between electrodes at the desirable small distances. More important, my method employs the capacitance and the conductance of the medium to first calculate the more useful complex permittivity. Golba and Hansen, in U.S. Pat. No. 4,448,943, do not utilize the complex permittivity of the reacting materials as I do.
Measurements through a wide range of frequencies are generally described in my article "Dynamic Dielectric Characterization of the Cure Process:LARC-160" (SAMPE Journal, July/August 1983, p. 18) and in my chapter "Electrical Methods of Characterization of Cure Processes in Resins" to be published in Developments in Reinforced Plastics-5 by Elsevier Publishers. However, many of the prior art dielectric or capacitance probes utilize either parallel plane electrodes or, if they are on the same plane, as mine are, they are either covered with insulation or the authors do not appreciate the importance of the dimension, geometry, and materials of construction as I have outlined. In this regard, the reader may be interested in reviewing Zurbrick et al U.S. Pat. No. 3,515,987 describing a dielectric probe employed illustratively as a moisture detector, Overall U.S. Pat. No. 3,873,927, also describing a "wet condition" detector, and Hanzawa et al U.S. Pat. No. 3,841,610, Geisselmann U.S. Pat. No. 3,777,257 and Burkhardt et al U.S. Pat. No. 4,057,823.
The Jung et al patent No. 4,296,630 shows a particularly clear illustration of an interdigitated pectinate configuration; however, the detector, being constructed to detect the presence or absence of a liquid, i.e. to detect the level to which the capacitor gauge is covered, is indifferent to the dimensions and material characteristics which are necessary to my purposes.
In most configurations of the prior art, the probe is in general a relatively massive and rigid structure. The configuration of electrodes is relatively or offhand constant. The particular materials used, the spacing of the electrodes, the width of the electrodes and the precision/reproducibility in the electrode pattern are not critical nor utilized as the purpose of these probes is generally to determine the presence or absence of a material or its proximity to the probe.
A number of workers in the art have employed field effect transistors and/or charge-flow transistors in probes or sensors to observe the electrical characteristics of various media. See, for example, Covington et al U.S. Pat. No. 4,437,969, Grudkowski et al 4,247,903, Janata et al U.S. Pat. No. 4,322,680, and a number of patents to Senturia and others, i.e. 4,317,084, 4,316,140, 4,209,796, 4,158,807, 4,236,121, and 4,423,371. A Senturia patent of particular interest is 4,352,059, showing, again, a configuration particularly for moisture measurements.